*4.3. Oxidation Resistance*

The oxidation testing confirmed that the non-reactive coating was characterized by the lowest adhesion to the substrates, as this was the only coating that experienced complete delamination (Figure 7). However, its oxidation behavior was superior to other coatings, because the as-formed oxide layer was dense and contained neither pores nor blisters. The sublimation of boron oxide is a known problem for refractory borides [50–52], so the lack of gas porosity in the Zr-Ta-Si-B coatings is remarkable. Despite increasing the adhesion, preventing the delamination, and enhancing the mechanical and tribological performance, the reactive sputtering clearly resulted in pronounced pore formation in the as-formed oxide layer upon annealing. The pores were formed by the oxidation of nitrogen- and carbon-containing phases to gaseous oxides. The blistering of the oxide layer is to be avoided if one strives to produce coatings with the highest possible oxidation resistance. The best trade-off between tribological performance and oxide layer porosity was achieved for coating 4. This coating was characterized by the lowest wear rate and propinquity to pore-formation upon oxidation among the investigated reactively deposited coatings. A further improvement might be achieved by the deposition of functionally graded coatings with the lower layer produced by reactive sputtering and upper layer deposited in Ar.

In addition, short-term annealing in air at 1200 ◦C for 10 min was also performed to assess the maximum operating temperature. It was found that the non-reactive coating completely oxidizes under these conditions. The thickness of the oxide layer for nitrogen-containing coatings 2 and 3 was about 8 microns (Table 2). Coating 5 with the maximum carbon content formed a 6.9 micron thick oxide layer. The lowest oxide thickness of 3.6 microns and the highest non-oxidized layer thickness of 6.2 microns were observed for coating 4 (Figure 8), in line with the results at 1000 ◦C.

**Figure 8.** *Cont.*

**Figure 8.** Cross-section SEM images and EDS maps of coatings 1 (Ar) (**a**), 2 (5 sccm N2) (**b**), 3 (10 sccm N2) (**c**), 4 (5 sccm C2H4) (**d**), and 5 (10 sccm C2H4) (**e**) annealed at 1200 ◦C for 10 min.

From the SEM-EDS data, it can be seen that a top-layer is formed on the surface of the coating, consisting mainly of silicon and tantalum oxides. Under the oxide film is an oxygen-free layer containing all the main elements of the coating, which has not undergone noticeable recrystallization.

Further analysis of oxidation mechanisms was based on XRD screening of coatings 2–5 annealed at 1200 ◦C for 10 min (Figure 9).

**Figure 9.** XRD patterns of coatings 2 (5 sccm N2), 3 (10 sccm N2), 4 (5 sccm C2H4), and 5 (10 sccm C2H4) after annealing at 1200 ◦C for 10 min: overview diffractograms (**a**) and enlarged areas (**b**).

A signal from alumina substrate can be recognized in all tested specimens after exposure to 1200 ◦C. A set of peaks that correlate well with (100), (101), (102), (110), (111), (200), (201), and (103) planes of the hexagonal h-TaSi2. The highest characteristic peaks h-TaSi2 (111) and (200) are located at 2Ө = 40.1◦ and 43.7◦, respectively. Intense peaks at 2Ө = 23.0, 28.3, 36.7, and 47.0 are related to oxidation product Ta2O5. In the case of nitrogen-rich coatings 2 and 3, the peaks for cubic TaN were found, with the highest peak at 2Ө= 41.7◦ (Figure 9b). The coatings deposited in C2H4 show the presence of (111) and (200) planes of FCC-TaC, with a corresponding highest peak at 2Ө = 34.8◦ (Figure 9b). The crystallite structure of h-TaSi2 calculated based on the broadening of (111) peak was 6–10 nm for all coatings. XRD patters corroborate that the reactive-sputtered coatings 2–5 are relatively stable and resistant to corrosion at temperatures up to 1200 ◦C.
